1. Field of the Invention
The invention pertains to a method of X-ray curing monomeric, oligomeric, or polymeric materials, such as molding materials, polymer composites, impregnated wood and adhesives.
2. Description of Related Art
Polymeric materials, including polymer composites, are used for a wide range of applications because of their excellent mechanical and structural properties and high strength-to-weight ratios. One way to initiate the curing process for polymers is to use an energy input.
When curing composites, the requisite energy is most often thermal, and is provided by an autoclave, oven, heated press, heated column or the like. However, thermal curing has several drawbacks, including long cure cycles, high energy consumption, stress induction due to temperature differentials within the object, expensive tooling, and often the emission of volatile organic compounds and greenhouse gases. Heat transfer efficiency drops as the thickness of the article increases, which imposes limitations on the thickness of the article and/or the processing time. Finally, for complex shapes, the molds employed must be carefully engineered to insure balanced thermal input.
In contrast to thermal energy, ionizing radiation causes very little heating of the material. X-ray curing is fast, energy efficient, environmentally friendly, safe and controllable. Accordingly, research on electron beam curing of reinforced plastics dates back to the late 1960's. W. Brenner, W. F. Oliver, Commercial aspects of instantaneous radiation cure of reinforced plastics. Society of Plastics Industry 22nd Annual Conference, Reinforced Plastics Division (1967). Similar work was carried on in the 1990's. A. Singh et al., Electron processing of fiber-reinforced advanced composites. Radiat. Phys. Chem. 48 (2), 153-170 (1996) and A. J. Berejka et al., Electron beam curing of composites in North America. Radiat. Phys. Chem. 63, 551-556 (2002). Illustrative methods are set forth in D. Lacour, et al, Method for Producing High Dimensional Precision Composite Elements Using Ionization Polymerization and Elements Produced by Said Method, U.S. Pat. No. 5,951,808, which describes methods to cure thin composite panels with high-energy electrons.
However, electron beams are severely limited in their penetration capabilities. For example, a 10 MeV electron beam, which is the highest practical energy due to the risk of generating radioactivity in certain materials, only penetrates about 2.5 cm of a typical composite when treated from one side. This puts limitations on the thickness and shape of parts that can be cured using electron beam energy and prevents the curing of such parts in molds. In addition, monomeric or oligomeric starting materials used as matrix binders, the high dose rates of electron beam processing can lead to an excess concentration of ionized species, thus creating disproportionation and termination reactions that compete with the desired polymerization and curing.
Wood has been impregnated with curable polymer forming reactants to impart surface hardness, dimensional stability and resistance to weathering. In such cases, thermal curing is undesirable due to the adverse effects heat has on wood and to the possible volatility of the reactants. Once again, electron beam curing is an alternative. However, such curing is limited to relatively thin products, such as flooring, due to the penetration limitations of electron beam processing. A. E. Witt Applications in Wood Plastics Radiat. Phys. Chem., vol. 9, nos. 1-3, 271-288 (1977).
Adhesives, such as epoxies, are commonly used to join similar or dissimilar materials. The adhesives can be cured thermally, but thermal curing imparts strain at the adhesive bond surfaces due to differences in the thermal expansion and contraction, and the thermal conductivity of the diverse materials. Once again, while electron beam processing has proven effective in thin film laminates, more complex shapes pose difficulty both in terms of beam penetration and orientation of the shaped article to the ionizing radiation source.
Gamma rays have been investigated as an alternative source of curing energy. However, gamma rays are emitted from a radioactive source at all times in all directions. Such emissions cannot be turned on or off, nor focused in a particular direction. Therefore, gamma ray exposure can be complicated and requires expensive and heavily regulated operating procedures.
Ultraviolet radiation (UV) has been investigated as an alternative source of curing energy. However, UV, while relying upon inexpensive sources, has even less penetrating power than electron beam. Ultraviolet radiation only provides line of sight surface curing.
The physical properties of high-energy X-rays are well known. This high-frequency electromagnetic radiation is produced when high-energy electrons strike any material. The X-ray yield increases with the electron energy and the atomic number of the target. The penetration in irradiated materials also increases with the electron energy. M. R. Cleland, X-ray Processing: A Review of the Status and Prospects, Radiat. Phys. Chem. 42(1-3), 499-503 (1993). The practical efficiency for converting incident electron beam power into emitted X-ray power is limited by basic physical considerations to about 8% with 5 MeV electrons and about 16% with 10 MeV electrons. J. Meissner et al., X-ray treatment at 5 MeV and above. Radiat. Phys. Chem. 57, 647-651 (2000). Such low power conversion efficiencies have heretofore implied low X-ray processing rates when compared to the radiation treatment of products that are thin enough to be treated with high-energy electron beams. Even with such low dose rates, there are some applications for X-ray processing where the greater penetration of high-energy X-rays have been found to be beneficial.
The European organization Aerospatiale, located in Saint Medard en Jallels, France, developed a process to cure carbon fiber composite cases for rocket motors with a combination of high-energy electrons and X-rays. Electrons were used on the thinner cross-sections and X-rays for thicker areas. These large structures were made by wet filament winding with carbon fibers coated with acrylated materials, which were then cured by irradiation in air. D. Beziers et al., Composites structures obtained by ionization curing, Radiat. Phys. Chem. 48 (2), 171-177 (1996). Daniel Beziers, Apparatus for the Polymerization and/or Crosslinking of a Resin Used in the Composition of a Composite Material Part by Means of Ionizing Radiation, U.S. Pat. No. 4,689,488, Aug. 25, 1987. Daniel Beziers, Process for the Polymerization and/or Crosslinking of a Resin Used in the Composition of a Composite Material Part by Means of Ionizing Radiation, U.S. Pat. No. 4,789,505, Dec. 6, 1988. The basic concepts of curing composites with X-rays has been reviewed briefly in another paper. D. R. Kerluke, et al., X-Ray Processing of Advanced Composites at 5 MeV and Above, in Proceedings of the SAMPE 2002 Conference, which was held in Long Beach, Calif., USA on May 12-16, 2002. SAMPE is an acronym for Society for the Advancement of Material and Process Engineering, Covina, Calif. 91722 USA.
Several firms have produced high-energy, high-power electron accelerators which have been used for various applications of X-ray processing. Nissin High Voltage, located in Kyoto, Japan, has made Cockcroft-Walton direct-current accelerators rated for 150 kW of electron beam power at 5 MeV. Radiation Dynamics, Inc. (RDI), a subsidiary of Ion Beam Applications, located in Edgewood, N.Y., USA makes Dynamitron® direct-current accelerators rated for 300 kW of electron beam power at 5 MeV. Ion Beam Applications (IBA), located in Louvain-la-Neuve, Belgium, makes Rhodotron® radio-frequency accelerators rated for 135 kW of electron beam power at 5 MeV and 200 kW at 7 to 10 MeV. IBA has recently developed and tested a more powerful Rhodotron rated for 500 kW of electron beam power at 5 MeV and 700 kW at 7 MeV. These very high power systems compensate for product through-put limitations which may occur with X-ray processing as a consequence of the power conversion inefficiencies for producing X-rays from electron beams. The basic concepts of Rhodotron® accelerators are described in Annick N'Guyen and Jacques Pottier, Electron Accelerator with Coaxial Cavity, U.S. Pat. No. 5,107,221, the entirety of which is incorporated herein by reference.